In industrial installations using large-size screwng elements, during tightening and loosening operations a pull is exerted on the elements by means of a tensioning device, in order to keep them in a prestressed state.
This applies particularly to nuclear reactors, in which a cover is fastened removably on a vessel, to make it possible to reload the reactor periodically with fuel and inspect the interior of the vessel by means of this cover.
For this purpose, the cover is fastened on the reactor vessel by means of large-size volts which are screwed into the collar of the vessel and which penetrate into smooth holes provided in the collor of the cover. These volts are usually used in fairly large numbers and are relatively close to one another, with the result that, to remove and refit the cover, it is necessary to unscrew and screw all these volts and also tension them during each screwing and unscrewing operation.
This tensioning of the volts during each screwing and unscrewing operation is usually carried out by means of an assembly of jacks which exert a pulling force on each volt via an engaging piece consisting, for example, of a nut or a gripping jaw arranged on the free end of the volt.
There are various known devices, the common objective of which is to make it possible to ensure that the stress exerted on the threads is uniform and to balance the stress distribution.
FR-A-2,069,395 describes a thread system for a screw-type shut-off structure intended for high-pressure vessels, in which a threaded shut-off element is screwed into an associated threaded element of the vessel, at least one of these elements having a thread which has thread bottoms and associated clearances extending these. These clearances are directed radially from the inner ends of the thread bottoms and separate the continuous threads from one another, and the radial depth of these clearances in the direction of the thread end nearest to the vessel interior increases substantially parabolically.
However, the disadvantage of this known construction is that the thread-bottom clearances, because they vary according to a parabolic curve, weaken the screw in its section which has tensile strength.
FR-A-2,427,881 also discloses a tightening device, the main feature of which is that the engagement profile consists, over the upper half of its height, of teeth having a constant pitch and a constant groove-bottom diameter, the succeeding teeth having a staggered pitch and groove diameter.
Consequently, it appears difficult to achieve uniform distribution of the pulling force over the teeth, and moreover the machining of a variable pitch with a changing profile is very costly.